1. Field of the Invention
This invention relates to a method of detecting flaws of an object of examination by using optical and electric (electronic) means and automatically determining the type and nature of each of the detected flaws.
2. Prior Art
Flaws on and/or in a product can seriously damage the performance of the product and significantly reduce its commercial value.
For instance, foreign objects contained in the insulation layers of rubber or plastic of a cable can significantly degrade the insulation breakdown characteristics of the cable.
While the probability with which foreign objects are introduced into insulation layers of rubber or plastic insulated cables has been greatly reduced with the recent development in the field of manufacture of insulated cables, there will still be a long way to go before flawless cables are produced.
Therefore, it is of vital importance to accurately analyze the insulation breakdown characteristics of each cable by examining the type, the size and the number of the flaws of the cable if it is marketed with a reliable degree of quality assurance.
In the case of an extra high voltage power cable, a minute foreign object that can be detected only by microscopic scrutiny using a microscope of a high magnification can seriously affect the insulation breakdown characteristics of the power cable if it is contained in any of its insulation layers. Therefore, a highly reliable inspection system should be established to cope with such problems, involving a large number of specimens to be examined, in order to provide an enhanced level of quality assurance for cables of this category.
The most dangerous foreign object is, of course metal debris. In order to safely eliminate metal debris from marketed cables, the type and nature of foreign objects should be determined before knowing the size and the number of the foreign objects for each type.
It is a common practice for examining the quality of a rubber or plastic insulation layer of an insulated cable to cut out a specimen having a thickness of 0.05 to 2 mm from the layer and observe the specimen through a microscope to visually determine the type, the number and the size of the flaws found in it.
Observation of a specimen proceeds in this technique proceeds in three stages: detecting flaws in the specimen, identifying each of the detected flaws by color and shape and metering the dimensions of each of the flaws (length.times.width.times.height).
Flaws in rubber or plastic insulated cables may normally grouped into three categories: ambers, black foreign objects and voids. Metal debris are regarded as black foreign objects.
When flaws are observed through a microscope that receives rays of light transmitted through the object of examination, each may take any of the following image patterns depending on the depth of observation along the center line of the microscope running through the focal point of the lens system.
Just: signifying that the depth of observation agrees with the focal length of the flaw. Under: indicating that the depth of observation is shallower than the focal length of the flaw. Over: meaning that the depth of observation is deeper than the focal length of the flaw.
These image patterns are summarized in Table 1.
Black foreign objects clearly appear black when the depth of observation is just and blurred and black when the depth of observation is under or over.
Ambers are hardly or not recognizable when the depth of observation is just because they turn totally white and do not make difference with the ambient resin color. They take on a black margin surrounding a white inside area when the depth of observation is over, whereas they are observed as totally black when the depth of observation is under.
Voids shows a black margin with the inside appearing white when the depth of observation is just, although they look just black when the depth of observation is under or over.
The effect of the above described technique of determining flaws by observation greatly depends on the skill of the operator working at the microscope and hence subject to deviations in terms of the accuracy of observation. Moreover, it is a time and labor consuming technique.
In an attempt to bypass this problem, there have been utilized automatic flaw detecting apparatus that are popularly used in other technological fields for detecting flaws of cable insulation layers.
With an apparatus of this type, flaws in a specimen of a cable insulation layer having an appropriate thickness can be automatically detected by differences in the level of brightness of the light transmitted through the specimen.
Referring to graphs (a) and (b) of FIG. 22 of the accompanying drawings, the apparatus may automatically recognize areas having a brightness lower than a predetermined threshold level X as flaws. Conversely, areas having a brightness higher than the threshold level X may be recognized as so many flaws by the apparatus.
This technique of automatic flaw detection is, however, also not without problems.
Firstly, when flaws are automatically identified by referring to the brightness of transmitted rays of light that can be higher or lower than a threshold level X, both voids that are over or under and ambers that are under appear black to the automatic flaw detector and it fails to discriminate them from each other and from black foreign objects.
Then, the net result will be incapability of meeting the requirement of classification of flaws before counting of the number and measuring the size of the flaws as in the case of microscopic observation by men.
Secondly, not only black foreign objects in the vicinity of the surface of the specimen but also scars on the surface (that can give rise to noise) can be recognized as black by the automatic detector if the threshold level X is held too high or low to enhance the sensitivity of the detector. Then, the detector will become totally powerless for flaw detection.
Thirdly, when the specimen has an uneven thickness, thin areas (bright areas) of the specimen can be recognized as white by the automatic detector.
Such white areas may be hardly discriminated from ambers which also look white when the depth of observation agrees with the focal length of the flaws.
As far as rubber or plastic insulated power cables are concerned, an irregular interface between a semi-conductive layer and an insulation layer can also provide causes of false recognition.
Some of the technological problems related to irregular interfaces and the currently available methods for determining irregularities on interfaces will be described below.
Referring to FIG. 15 of the accompanying drawings, which illustrates in cross-section a rubber or plastic insulated power cable having an inner semi-conductive layer 11, an insulation layer 14 and an outer semi-conductive layer 12, a tree (ramified crack) can develop in the insulation layer 14 if there is an irregular area in the interface 15 of the inner and/or outer semi-conductive layer 12 and the insulation layer 14 as the electric field generated in and around the cable shows a high degree of concentration there.
A flaw of this type can also lead to a degraded performance of the cable and eventually break down the insulation layer 14.
In view of the current circumstances for power cables, where rubber or plastic insulated cables and particularly bridged polyethylene insulated cables are used for extra high voltage applications involving voltages as high as 2,75 KV, cables of this type need to be carefully and microscopically scrutinized by using a large number of specimens per each cable so that any irregularities on interfaces of layers may be rigorously checked.
A known practice of microscopically examining specimens to meet rigorous requirements for high voltage applications of sheathed cables is taking specimens (such as an object of examination 17 in FIG. 16) having a thickness of approximately 0.5 mm out of a cable by means of a microtome and then visually scrutinizing them through a microscope.
Assume here that a specimen as shown in FIG. 16 is prepared by means of a microtome. Of the specimen 17 of FIG. 16, only a limited small area 18 may be observed by a single operation of microscopic examination to produce an image, for instance, as shown in FIG. 17(A).
In FIG. 17(A), the shadowed area 11 or 12 is an opaque inner or outer semi-conductive layer of the object of examination 17 and the white area 14 indicates a transmissive insulation layer, the interface 15 of the two layers being also shown as a shadowed area.
The inner or outer semi-conductive layer 11 or 12 is tapered along the interface 15 as seen from FIG. 17(C), which is a sectional view along Y.sub.1 --Y.sub.1 line of FIG. 17(A).
An irregularity 16 found on the interface 15 is in fact part of the inner or outer semi-conductive layer 11 or 12 semispherically projecting toward the insulation layer 14 as shown in FIG. 17(C), which is a sectional view along Y.sub.2 --Y.sub.2 line of FIG. 17(A).
Then the detected irregularity 16 may be measured for its width W, height H and surface area as illustrated in FIG. 17(A). The number of irregularities in the specimen 17 may also be counted.
The above described procedure of detecting and identifying flaws (interface irregularities) is time consuming particularly when a large number of specimens are involved because the width and height of each flaw can be very small and normally found to be around several .mu.m.
Moreover, the reliability such a flaw checking technique may be questioned because it relies heavily on the human vision while minute irregularities of the order of micromillimeters need to be controlled to meet the current technological requirements.
There has been proposed a technique that utilizes the phenomenon that irregularities normally take the form of a semisphere as shown in FIG. 17(C) and present a shady graduation when they are magnified. In a detector realized by using this technique, a threshold value is set for detecting dark areas of irregularities and an electric signal is generated each time it detects an irregular spot.
While this technique may provide some help for flaw detection, it is not capable of identifying the boundary line of an irregular spot existing on the interface of a semi-conductive layer and an insulation layer because it can only detect the darkest areas of irregularities that exceed a preset threshold level of darkness.
In short, there have not been practically feasible methods and apparatus capable of automatically detecting irregularities on various interfaces.